1. Field of the Invention
This invention relates, generally, to the art of lasers. More particularly, it relates to a lens for high power lasers.
2. Description of the Prior Art
One or more high quality fused silica lenses are used to focus high power fiber lasers in excess of one kilowatt (1 kW). More specifically, at least one high quality fused silica lens is used to collimate laser light emitted from a fiber laser where the fiber ranges in diameter from 50 microns to over 300 microns. After it has been collimated, the light is directed to a focusing lens assembly made of one or more high quality fused silica lenses that focus the light on to a surface to be cut, drilled, scribed, marked or welded.
Although the fused silica lens material is highly transmissive, some radiation is either absorbed or scattered within the lens, causing the lens to heat up. All optical glass materials have certain thermal properties that change the focus characteristics of a lens as they heat up. In particular, the thermal coefficient of expansion α and change in index of refraction n as a function of temperature (dn/dT) alter the power of a lens. The power of a lens is influenced by these two properties and is called the thermal power of a lens:
      Ψ    p    =      [                                        ⅆ            n                    /                      ⅆ            T                                    (                      n            -            1                    )                    -      α        ]  
The power of a lens is therefore altered as a function of temperature by the following equation:
      ΔΦ    =          Φ      ⁡              [                                                            ⅆ                n                            /                              ⅆ                T                                                    (                              n                -                1                            )                                -          α                ]              ,where Φ is the power of the lens. The change in power then is the original power of the lens times the thermal power of the lens Ψp.ΔΦ=ΦΨp 
Fused silica has a very low coefficient of thermal expansion, a very high transmission throughout the ultraviolet to near infrared wavelengths of the electromagnetic spectrum, and low scattering qualities. It is currently the most cost effective glass for the task. However, as indicated by the above equations, it is, along with all other optical glasses, susceptible to focal power changes as the temperature of the glass increases. The problem of focal power changes due to heat is problematic with conventional fiber lasers having average powers now in excess of 20 kW.
The coefficient of thermal expansion (CTE) for fused silica is about 0.5×10−6/° K and has a dn/dT of ˜10×10−6/° K. A fused silica lens having a nominal focal length of 200 mm will have a change in focus of more than 350 microns for a 100° C. temperature increase. Although this is not a huge amount for such a long focal length lens, when used as a collimator for a fiber it has a substantial impact on how the light is collimated from the fiber.
A paper written by Abt et al, Focusing High-Power, Single Mode Laser Beams, Photonics Spectra Magazine, May 2008, discusses this problem and shows focus shifts of between 1 and 2 mm through a range of powers of 100 watts to 900 watts of laser power using a variety of fused silica lens and gradium index glasses. Steele et at describe a similar behavior with a CO2 laser in Spot Size and Effective Focal Length Measurements for a Fast Axial Flow CO 2 Laser, in a paper released by the Department of Energy. Thermal lensing in window materials is further discussed by Klein in Materials for High-Energy Laser Windows: How thermal Lensing and Thermal Stress Control Performance, SPIE Proceedings Vol. 6666, 66660Z1 (2007).
The conventional method for handling thermal lensing of an optical system is to let the system thermally stabilize for three or four minutes, followed by readjusting the focus of the collimation optics and the focusing optic. This is a very undesirable and expensive delay in a production environment.
U.S. Pat. No. 5,128,953 discloses a method for aiding the cooling of a lens by placing a small gap between the focus lens and a debris shield. This method has utility with low power lasers. It does not solve the collimation problems of high power fiber lasers without adding additional optics which should be avoided with high power lasers. The prior art requires an additional window, i.e. an extra optic, without improving the optical performance and without providing a means to provide a cooling gas when a multi element lens is required.
European patent application EP 1 791 229 A1 discloses a method for reducing thermal lensing with the use of radially polarized light and stress birefringence in ZnSe. This approach has very limited utility and is not practical for high power fiber lasers which are not polarized.
Athermalization is commonly applied to mid infrared optical systems but not specifically applied to high power lasers. These systems are adapted to compensate thermal changes over a broad spectrum within the infrared. Athermalization is discussed by Smith, Modern Optical Engineering, McGraw Hill 2000 and in Practical Optical System Layout and Use of Stock Lenses, McGraw Hill, 1997 and by Fishcer et al in Optical System Design, McGraw Hill, 2008. These texts teach to achromatize and athermalize by solving three simultaneous equations:
      Φ    =                  Φ        a            +              Φ        b                  ΔΦ    =                            φ          a                ⁢                  Φ          a                    +                        φ          a                ⁢                  Φ          b                                        ⅆ        Φ                    ⅆ        T              =                            Ψ          a                ⁢                  Φ          a                    +                        Ψ          b                ⁢                  Φ          b                    
Where Φ is the power of the lens system, Φa and Φb are the powers of the individual lens elements; φa and φb are the chromatic powers of each element and Ψa and Ψb are the thermal powers of the lenses. The chromatic power of a lens is the inverse of its abbe value v.
In view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the prior art could be overcome.